Dilute-Nitride Low-Dimensional Nanostructures Formed on Non-Planar Substrates

The properties of semiconductors heterostructures of nanoscopic dimensions change from that of bulk material according to the rules of quantum mechanics. The planar quantum wells (QWs) are widely used in various diode and laser devices thanks to the relative ease of fabrication and to their improved electronic and optical performance compared to bulk materials. Quantum effects become more apparent when the charge carriers are confined in more than 1 spatial dimension. Much scientific interest was initially dedicated to the quantum wire (QWR) structures, which confine carriers into a quasi-1D space. But their sensitivity to disorder and the development of efficient fabrication methods of quantum dots (QD) shifted the attention to this latter system. The carrier confinement in the three directions of space confers to these structures a discrete spectrum of energy states, with the state occupancy ruled by the Pauli exclusion principle. The prospective applications are numerous in domains such as ultra-low threshold lasers, quantum cryptography, true random numbers generation, quantum electrodynamics experiments, etc. One of the prominent semiconductor growth techniques is the metalorganic vapour phase epitaxy (MOVPE). We used this method to produce ordered QWRs and QDs at the bottom of V-shaped and tetrahedral recesses, respectively. These nanostructures form by the complimentary actions of nano-capillarity and growth rate anisotropy in these recesses etched in GaAs substrates. This fabrication process offers some key advantages over other methods. The emission energy is very well controlled, with a narrow inhomogeneous broadening and a high uniformity across the wafer. Combined with the highly precise control on the formation site, this offers the possibility of high-yield integration of one or even several nanostructures, e.g., into photonic crystal devices. Other advantages of this approach are the impressive tunability of the electronic potential within the nanostructures and the possible use of well-defined intraband transitions. However, the emission energy of V-groove QWRs and pyramidal QDs studied so far is quite limited due to the formation mechanism imposing a low degree of strain. Incorporation of nitrogen has dramatic effects on the band structure of GaAs-based materials. Dilute concentrations (< 5 %) lead to a huge shrinkage of the bandgap, as well as to large changes in various other electronic properties. Addition of nitrogen into QWRs and QDs nanostructures may thus allow emission wavelengths above 1 μm, possibly up to the 1.3 μm telecommunication window of silica optical fiber. In view of these opportunities, the first part of this work is dedicated to the study of N incorporation into QWs grown on vicinal-(100) GaAs substrates. The substrate miscut angle is often neglected in growth studies, and is of great importance in the cases of V-grooves and pyramidal recesses because of the large misorientations of the recesses facets. In our studied dilute-nitride QWs grown by MOVPE, large emission redshifts are achieved (> 250 meV). The substrate miscut and the surface corrugations are shown to play an important role in the N incorporation efficiency: QWs grown on large substrate misorientations emit at longer wavelength than those grown on usual (100)-"exact" substrates, while exhibiting a comparable luminescence efficiency. The importance of a uniform N distribution within the QW is stressed, which appears difficult to achieve when the effect of surface corrugation is combined with that of In segregation. The second part of the work focuses on the N incorporation into V-groove QWRs. Important emission redshifts are achieved, in the ∼ 250 meV range. We first detail the emission spectrum and assert the 1D-character of the carrier wavefunctions. The influence of various growth and structural parameters is explored, leading to the fabrication of QWRs emitting at 1.3 μm at room temperature. The evolution of the polarization properties with temperature is also characterized. The third main topic and primary goal of this thesis is the nitrogen incorporation into QDs formed in inverted pyramids etched on (111)B GaAs substrates. A study is first conducted to understand the effects of several growth and structural parameters on the emission properties of InGaAs QDs. Nitrogen incorporation into QDs is then successfully demonstrated. The monitoring of the lateral QWR emission energy suggests a peculiar N incorporation pattern, or a significant perturbation of the formation of these lateral nanostructures. By contrast to what achieved with QWs and QWR structures, only limited emission redshifts were achieved (∼ 75 meV). The QD linewidths, degree of linear polarization and fine structure splitting are significantly deteriorated when compared to the InGaAs counterparts. These results cast serious doubts on the perspective of high-quality GaAs-based QDs in pyramids emitting at long wavelength. Our results demonstrate that nitrogen does not have the potential to shift the emission wave-length of InGaAs pyramidal QDs up to 1.3 μm, while simultaneously satisfying strict quality requirements. But other material systems may offer such opportunities. We briefly explore the possibilities of growing InGaAs/InAlAs nanostructures on patterned InP wafers. This ongoing project may open new possibilities for exploiting the pyramidal QD system. A kinetic Monte-Carlo numerical algorithm was implemented, reproducing by deposition and diffusion processes the evolution of the pyramidal template during growth. The numerical experiments were compared with post-growth AFM measurements of real samples. The recesses are observed to strongly affect the monatomic step flow on the neighboring (111)B surfaces. The simulations especially evidence the strong attraction of the pyramid apex on the atoms of the surrounding area, tending to elevate the QD formation site from the nominal C3V symmetry toward a hexagonal one.